1. Field of the Invention
The present invention relates to a thrust bearing structure configured to bear a thrust load applied in an axial direction, and a supercharger equipped with the thrust bearing structure.
2. Description of the Related Art
A supercharger known as an example of rotary machines includes a housing, a rotor shaft rotatably provided to the housing, a compressor impeller attached to one end of the rotor shaft, and a turbine impeller attached to the other end of the rotor shaft. The compressor impeller compresses air by using a centrifugal force while the turbine impeller generates torque by using pressure energy of an exhaust gas. The compressor impeller and the turbine impeller are connected to each other through the rotor shaft and these impellers therefore rotate as a unit. At the time of the rotation, the rotor shaft receives loads applied not only in a rotating direction but also in an axial direction. For this reason, the supercharger is equipped with a thrust bearing structure configured to bear the load in the axial direction (so called a thrust load).
Concerning the thrust bearing structure, Japanese Patent Application Laid-open Publication No. 2002-364635 discloses a structure that utilizes a wedge effect of lubricating oil. A thrust bearing structure of PTL 1 includes a thrust collar and a thrust bearing. The thrust collar and the thrust bearing are each formed into an annular shape and are arranged facing each other in the axial direction of the rotor shaft. The thrust collar is fixed to the rotor shaft and rotates as a unit together with the rotor shaft.
Meanwhile, the thrust collar includes a collar pad on one surface thereof. A surface of the collar pad is orthogonal to a shaft center of the rotor shaft. On the other hand, the thrust bearing includes multiple bearing pads on a surface thereof facing the collar pad. Each bearing pad includes a tapered section and a land section. A surface of the tapered section is inclined in such a manner as to gradually get closer to the collar pad in the rotating direction of the rotor shaft. In other words, the tapered section is formed to have a wall thickness that gradually becomes thicker in the rotating direction of the rotor shaft. A surface of the land section is orthogonal to the shaft center of the rotor shaft and continuous with the leading end of the tapered section in the rotating direction. That is to say, when the collar pad of the thrust collar and the bearing pad of the thrust bearing face each other, the clearance therebetween becomes smallest at the land section.
An interspace between the collar pad and the thrust bearing is filled with lubricating oil. When the rotor shaft rotates together with the collar pad, the lubricating oil flows in their rotating direction due to the rotation and viscosity of the lubricating oil. Meanwhile, as described above, the clearance between the collar pad and the bearing pad becomes smallest at the land section. Hence, the flow of the lubricating oil converges between the land section and the collar pad. As a consequence, a pressure inside the lubricating oil is increased at the land section and at a portion of the tapered section near the land section, thereby drawing the collar pad and the thrust bearing away from each other. This effect is generally referred to as a wedge effect. While the supercharger is in operation, the thrust load acting on the rotor shaft can be borne by this wedge effect.